This invention relates to both a method of manufacturing tubular elements utilizing filament winding, and the products resulting from such manufacturing.
The use of fiber-reinforced composites in the construction of articles and structural parts has dramatically increased in recent years, and now includes the manufacture of tubular elements and shafts for use, for example, as ski poles, spars for hang gliders and the like, golf shafts, rocket launch tubes, mechanical drive shafts, etc. The reason for this, of course, is the high strength-to-weight ratio achievable with fiber-reinforced composites.
The most commonly used method in manufacturing fiber-reinforced composite tubular shafts is the so-called hand lay up process, carried out either by hand or hand operated machine. In this process, the fiber or filament is formed into strips of tape and then pre-impregnated with resin (or a polymer). The resin is then "staged," or partially cured, after which backing paper is applied to the strips of tape. The pre-preg tape, as it is called, is then ready for storage and/or shipment under refrigerated conditions.
For actual manufacture of the tubular elements with various fiber orientations, the pre-preg tape is cut into wedge-shaped strips leaving, of course, unusable "cookie cutting" leftover portions. The backing paper is removed from the wedge-shaped strips which are then placed onto mandrels either by hand or with a pressure rolling machine, one tube or shaft at a time. Multiple layers of the tape are placed onto the mandrel, all the time attempting to keep track of and locate the seams evenly about the mandrel, until the desired thickness is achieved.
Compacting of the pre-preg tape onto the mandrel is typically then carried out by applying cellophane shrink tape over the surface of the tape layers by a high pressure applicator, after which the tube or shaft is oven cured. After the fiber-reinforced composite material is cured, the mandrel is extracted from the shaft and the cellophane tape is removed (by a scrubber machine). The tube or shaft is then finished by cutting the ends, smoothing the exterior surface by sanding or other grinding method, and coating the surface with urethane or paint.
Another method used in forming the fiber-reinforced composite tubes or shafts involves wet-winding the filament onto mandrels. After the winding is completed, the other steps outlined above for the hand lay up process must also then be carried out for the wet-wound tubes or shafts.
The two above-described methods of manufacturing tubes or shafts are both labor intensive, require a significant amount of manual handling and processing, result in much wasted material, and yield a tube or shaft whose exterior surface generally requires additional finishing, and whose straightness depends on the straightness of the mandrel used.
One method of compacting a composite tube or shaft wrapped on a tapered mandrel, and of curing the composite, is disclosed in U.S. Pat. No. 3,896,206. In this method, the mandrel (with layers of fiber pre-impregnated with resin wound thereabout) is placed in a two-piece mold having an elongate tapered cavity. The mandrel is then forced into the mold to thereby force the composite layers against the sidewalls of the cavity, and compact the composite material onto the mandrel. Although this method serves to provide a smoother finish for the tube or shaft than the earlier-mentioned two methods, parting line marks would still be present on the exterior because of the use of the two-piece mold, and the extensive manual handling and processing for wrapping the pre-impregnated strips on the mandrel would still be required.